Best ap for changing partition sizes ?

Cloning before modifying is certainly a good idea :-)

But the process is very simple -- you just need to do 3 steps:

(1)  Resize D: to free up the space you need.
(2)  "Slide" D: so the free space is between C: and D:
(3)  Resize C:

Done :-)

You can easily do these with the free demo version of Boot-It BM  [http://www.terabyteunlimited.com/bootit-bare-metal.htm ]

Download it;  extract the files;  create a bootable CD with the included MakeDisk utility;  then do the following ....

(a)  Boot to the CD; selecting CANCEL at the first prompt; then OK to go to Maintenance Mode.

(b)  Click on Partition Work

(c)  Highlight the 2nd partition (this corresponds to D: -- it should be obvious from the size.   Then click on ReSize and make it the desired size.    This operation should be fairly quick (VERY quick if there's not much data on D:, as you implied)

(d)  With the same (now resized) partition highlighted, click on Slide ... and choose 0 free space "After".     Wait for this to complete => this is the longest step, as it needs to move all of the data on D:

(e)  Now highlight the first partition (corresponding to C:);  and click on ReSize.   Select the new size (the max available) and let it finish [This will be VERY quick].

Done !!    Just click on Close, then on Reboot => and the system will reboot to XP.    XP will "find new hardware", as it thinks the drive has changed (it's now larger) ... but after a 2nd reboot you're done.

Here's an outline of what you're doing pictorially:

Your disk now looks like this:
CCCCCCCCCCDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDD

After you resize D:, it looks like this:
CCCCCCCCCCDDDDDDDDDDDDDDDDDDDxxxxxxxxxxxxxxxxxxxxxxxxxx
... where the x's are free space

But you can't add the free space to C:, because D: is "in the way"

After the "Slide", the disk looks like this:
CCCCCCCCCCCxxxxxxxxxxxxxxxxxxxxxxxxxxDDDDDDDDDDDDDDDDDD

THEN you can resize C:, and the disk looks like this:
CCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCDDDDDDDDDDDDDDDDDDD